CDC officials said the risk to other passengers on the plane - identified as US Airways Flight 401 - was minimal because the flight was less than eight hours.

"When transmission does occur, it tends to occur on long-haul flights," longer than eight hours, said Dr. Martin Cetron, CDC director for global migration and quarantine. "Fortunately, we were able to intercept this individual before they took the high-risk flight."

The patient, who is not being identified, was added to a federal "do not board" list on Friday. It's unclear how the man managed to get on the US Airways flight the next day. An investigation is under way.

The federal "do not board" list was created in June 2007 to prevent the spread of contagious diseases like tuberculosis. Since then, 88 people have made the list, all of them infected with tuberculosis, Cetron said.

People are put on the "do not board" list if they are contagious and are considered likely to try to get on an airplane, even if they've been told they shouldn't fly.

"Often individuals have, from their perspective, a compelling reason to want to travel, maybe to visit a sick family member back home," Cetron said. "So there is this tension between the individual's interest and needs and the broader needs of the public at large."

Tuberculosis is a bacterial infection that usually attacks the lungs. Most people infected with tuberculosis are not actively sick with it. In active cases, symptoms include a persistent, long-lasting cough, fever and night sweats.

Drug-resistant cases of tuberculosis, which are more challenging to treat than nonresistant TB, are becoming more common worldwide. San Francisco public health officials said the man on Saturday's flight has a drug-resistant form of tuberculosis.

"If someone is untreated and they're coughing, they are putting the infectious organisms in the air. They shouldn't be going into enclosed environments," said Dr. L. Masae Kawamura, director of San Francisco's TB Control Section. "Going on a plane, that's not OK until you're safe to be back in the public."